Baseband Demodulation/Detection Chapter 3 Baseband Demodulation/Detection

Baseband Demodulation/Detection 2

Demodulation and Detection

Why Baseband Demodulation/Detection ? Received pulses are distorted because of the following factors: Intersymbol Interference causes smearing of the transmitted pulses. Addition of channel noise degrades the transmitted pulses. Transmission channel causes further smearing of the transmitted pulses. Demodulation (Detection) is the process of determining the transmitted bits from the distorted waveform. Transmitted Received waveforms as a function of distance waveform 4

Models for Transmitted and Received Signals For binary transmission, the transmitted signal over a symbol interval (0, T) is modeled by The received signal is degraded by: (i) noise n(t) and (ii) impulse response of the channel for i = 1, …, M. Given r(t), the goal of demodulation is to detect if bit 1 or bit 0 was transmitted. In our derivations, we will first use a simplified model for received signal Later, we will see that degradation due to the impulse response of the channel is eliminated by equalization.

Basic Steps in Demodulation

A Vector View of CT Waveforms (1) Orthonormal Waveforms: Two waveforms y1(t) and y2(t) are orthonormal if they satisfy the following two conditions Normalized to have unit energy 2. Two arbitrary signals s1(t) and s2(t) can be represented by linear combinations of two orthonormal basis functions , i.e.

Geometric Representation of Signals

A Vector View of CT Waveforms (2) 1. N-dimensional basis functions: consists of a set {yi(t)}, (1 ≤ i ≤ N), of orthonormal (or orthogonal) waveforms. 2. Given a basis function, any waveform in the represented as a linear combination of the basis functions.

A Vector View of CT Waveforms (3) Given a basis function, any waveform in the represented as a linear combination of the basis functions or, in a more compact form, The coefficient aij are calculated as follows where Kj is the energy present in the basis signal.

Activity 1 Demonstrate that signals si(t), i = 1,2,3, are not orthogonal. Demonstrate that yi(t), i = 1,2, are orthogonal. Express si(t), i = 1,2,3, as a linear combination of the basis functions yi(t), i = 1,2. Demonstrate that yi(t), i = 1,2, are orthogonal. Express si(t), i = 1,2,3, as a linear combination of the basis functions y i(t), i = 1,2. 1111

Activity 1 (a) s1(t), s2(t), and s3(t) are clearly not orthogonal, i.e. the time integrated value over a symbol duration of the product of any two of the three waveforms is not zero, e.g. (b) Using the same method, we can verify this signal form an orthogonal set.

Activity 1 (c) We know that

Activity 1 (d) are obviously orthogonal. (e) Similar to (c), we have

Activity 2 Show that the energy in a signal si(t) is given by

Activity 2

SNR used in Digital Communications In digital communications, SNR is defined as the ratio of the energy (Eb) present in the signal representing a bit to the power spectral density (N0) of noise. In terms of signal power S and the duration T of bit, the bit energy is given by Eb = S × T. In terms of noise power N and bandwidth W, the PSD of noise is given by N0 = N / W. SNR is therefore given by where Rb is the rate of transmission in bits transmitted per second (bps). Bit-error probability is the probability of error in a transmitted bit. ROC curves are plots of Bit-error probability versus SNR.

Detection of binary signal in AWGN

Maximum Likelihood Detector (1) The sampled received signal is given by where ai(T) represents the signal levels obtained after sampling. For bit 1, ai(T) = a1 and for bit 0, ai(T) = a2. The pdf of n0 is Gaussian with zero mean The conditional pdf of z given that bit 1 or bit 0 was transmitted is given by

Maximum Likelihood Detector (2) The conditional pdf of z given that bit 1 or bit 0 was transmitted are referred to as maximum likelihoods. The maximum likelihoods have the following distributions

Maximum Likelihood Detector (1) Maximum Likelihood Ratio Test: is given by where P(s1) and P(s2) are the priori probabilities that s1(t) and s2(t), respectively, are transmitted. H1 and H2 are two possible hypotheses. H1 states that signal s1(t) was transmitted and hypothesis H2 states that signal s2(t) was transmitted. For P(s1) = P(s2), the maximum likelihood ratio test reduces to

Activity 3 Show that the probability of bit error for maximum likelihood ratio test is given by Values of Q(x) are listed in Table B.1 in Appendix B page 1046. 22

Matched Filtering (1)

Matched Filtering (2) Design the Receiving filter h(t) Design a filter that maximizes the SNR at time (t = T) of the sampled signal The instantaneous signal power to noise power is given by where ai(t) is the filtered signal and is the variance of the output noise The information bearing component is given by

Matched Filtering (3) Given that input noise n(t) is AWGN with Sn(f) = N0/2, the PSD of the output noise is given by The output noise power is given by The SNR is given by

Matched Filtering (4) Schwartz Inequality: with the equality valid if f1(x) = kf2*(x). Applying the Schwatz inequality to the SNR gives or,

Matched Filtering (5) The maximum SNR is given by which is possible if

Activity 4 Given that the transmitted signal is shown in the following figure, determine the impulse response of the matched filter. 28

Matched Filtering (6) Correlator Implementation of matched filter: The output of the matched filter is given by The output at t = T is given by which leads to the following correlator implementation for matched filter

Detection of binary signal: Review Matched Filter: Impulse response: Maximum SNR = (a1)2/s20 = 2Es/N0 ML Detector: Probability of Error: The overall goal of the receiver should be to minimize the probability of bit error PB.. In other words, we are interested in maximizing (a1 – a2)2/s20. The filter is designed such that it is matched to the difference of [s1(t) – s2(t)]. Maximum SNR of the matched filter = (a1 – a2)2/s20 = 2(Es1 – Es2)/N0 = 2Ed/N0. Probability of Error:

Activity 5 By defining the cross-correlation coefficient as show that the probability of bit error using a filter matched to [s1(t) – s2(t)] is given by Using the above relationship, show that the probability of bit error is given by

Activity 6 Determine the probability of bit error for a binary communication system, which receives equally likely signals s1(t) and s2(t) shown in the following diagram Assume that the receiving filter is a matched filter and the power spectral density of AWGN is N0 = 10-12 Watt/Hz.

Error Probability Performance of Binary Signal 1. Unipolar signaling In Unipolar signaling, the signal selection to represent bits 1 and 0 is as follows: The receiver is shown by the following diagram.

Unipolar Signaling Unipolar signal forms an orthogonal signal set. When s1(t) plus AWGN being received, the expected value of z(T), given that s1(t) was sent, is When s2(t) plus AWGN being received, a2(T)=0. The optimum decision threshold is: The bit-error performance is:

Bipolar Signaling In bipolar signaling, the signal selection to represent bits 1 and 0 is as follows: The receiver is shown by the following diagram.

Bipolar Signaling Bipolar signal is a set of antipodal signal, e.g. s1(t)=-s2(t). The optimum decision threshold is: The bit-error performance is:

Unipolar vs. Bipolar 37